Self-funded PhD- Population biology of coconut crabs

University of Bristol

Population biology of coconut crabs

Anthropogenic change has led to highly fragmented habitats but widely dispersed disjointed populations living there are a challenge to monitor because sub-population sizes may be very small, difficult to access, and time consuming to sample regularly. Our group has used the coconut crab (Birgus latro) on Pemba, the northern-most island archipelago of the United Republic of Zanzibar, as a case study for estimating highly fragmented populations and meta-population sizes. The species is a very large terrestrial decapod threatened by exploitation and habitat alteration and now classified as vulnerable. Coconut crabs are also found on outlying islands around Unguja, the southern Zanzibar island (Hamad et al 2023). This PhD study would attempt to estimate sub-population sizes of this species on 15 outlying islands around Unguja some of which are slated for hotel development. The work involves developing an integrated model to analyse capture-mark-recapture (CMR) and count data to estimate site-level densities and sub-population sizes and predict total population sizes. Simultaneously, the effects of various human influences would be investigated from a comparative perspective for example, agriculture, hotels, proximity of villages and so on (Caro et al 2023). The goal is to make a total population estimate for Zanzibar and to identify the particular islets that require conservation effort.

In addition, coconut crabs show marked colour polymorphism, the function of which is unknown (Caro & Morgan 2020). Currently we think that the red/blue coloration is found at a 3:1 ratio everywhere suggesting a simple genetic signature, but we suspect that founder effects may skew the polymorphism to one colour or another in fragmented populations. This project would explore the effects of population sizes and distance from the main island (immigration) at which founder effects come into play and hence the way that habitat fragmentation structures morphological traits.  

This project will train a student in techniques of population monitoring, standardized photography at night in the field, utilization of landsat imagery, statistical modelling and potentially becoming involved in a conservation awareness campaign.

The applicant needs to be willing to conduct fieldwork at night, camp on small islands in Zanzibar or stay in people’s houses, be comfortable on very small boats, can swim, and know or be willing to learn Swahili to fluency. Knowledge of R and willingness to learn about statistical modelling is required.

Entry requirements

Applicants must hold/achieve a minimum of a master’s degree (or international equivalent) in a whole-organism biological science that involved fieldwork. Applicants without a master’s qualification may be considered only on a very exceptional basis.

How to apply 

Before applying, please read carefully the information on the prospectus Biological Sciences Study at Bristol University of Bristol and make sure you have all the documents listed in the Entry Requirements – Admissions Statement and English Language Requirements.  

To apply, follow the link Start your application Study at Bristol University of Bristol and select the programme “Biological Sciences (PhD)”.

If you have questions about the project, please contact Prof. Tim Caro at

Professor Tim Caro – Our People (

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